There are many motor designs which have emerged over the years. Of particular interest is a type of motor whose stator resides on the outside and rotor resides on the inside of the motor structure. Sometimes this is called an “inside runner” since the moving element is on the inside and the stationary part (stator) is on the outside.
In most motors, the electrical-current-carrying conductors are made of copper, so throughout this disclosure, the word “copper” and “turns of copper” are used to describe the makeup of the coil. However, this should not be deemed a limitation, since some motors use aluminum or even silver wire to carry the electrical current. Moreover, it should be understood that the wire used in motors is insulated, so that subsequent turns do not short out with the rest of the coil, and that each turn does not short out to slots in which the turns are placed. In most cases, copper magnet wire is used, which is insulated with varnish, but the insulation can be anything that prevents electrical contact, such as cloth or even oxides.
There is a figure of merit in motor design called the “Motor Constant”, which is designated with the letters KM. The Motor Constant is a measure of the amount of torque produced compared with the power (i.e. heat) dissipated during the production of that torque. KM is expressed in terms of Torque per square root of watt, but may also be found by dividing Torque Constant (KT) by the square root of coil resistance. In all motors and actuators, the more copper you can fit into a given area, the greater the KM will be, and thus, in high performance motors, it is always desirable to maximize the amount of copper that is placed into the winding area.
In high performance motors, it is also desirable to effectively remove any heat that is generated by the windings. Coincidentally, the way you do this is also by maximizing the amount of copper that is placed into the winding area.
Copper has almost the highest thermal conductivity of any material, and thus, when turns of copper are placed close to one another, these turns can share the heat and also help to dissipate the heat to the stator material. Material other than copper (such as air or insulation) located in between the turns will dramatically reduce the heat capacity of the motor.
Typical coils are usually wound in a spiral fashion, starting at the inner-most radius, and arranging turns of wire side-by-side (for example from left to right). The number of turns arranged side-by-side establishes the “thickness” of the coil. Coils may have more than one layer, in which case, once all of the turns are arranged side by side for the first layer, this direction must reverse (for example from right to left) while turns are placed side-by-side on the next layer. Additional layers establish the “width” of the coil.
It is typical for the coil to be wound around an object that establishes its interior shape. The interior shape (i.e. the coil's inner radius) may be round, square, oval or practically any convex shape. The object around which a coil is wound may have “side-walls” that determine the thickness of the coil, and help to retain the wire during the winding process. The object around which a coil is wound is often referred to as a “bobbin” or a “coil former.” Throughout the rest of this document, we refer to it as the coil former.
Once the coil is wound around the coil former, the coil and its former may result in a single assembly that remains together for the rest of their lives. For example, it is common for coils to be wound around a plastic coil former (bobbin), and then laminations to be inserted around this coil/former assembly to create a transformer. In this case it is clear that the coil and its former remain together after the winding process.
In other coils, the coil former is removed once the coil is wound. This is most often done in what are called “self-supporting coils”. In order for a “self-supporting coil” to retain its shape, an adhesive must be used either after the winding process, or even during the process. It is known in the art to use a special kind of magnet wire called “bondable wire”, which has an adhesive layer as a part of the wire. Once a self-supporting coil is wound on a former, the bonding layer is activated, either by heat or by solvent or both.
In the field of coil winding, there is a term called “nesting”, which refers to the way in which the individual turns of wire are arranged with respect to one another. It is well known that, when using round wire (the most common type) to create a coil, ideal nesting happens when the turns on each layer are wound right next to one another, and the turns on subsequent layers are wound in the groves created by the turns from the previously-wound layer. FIG. 1 illustrates one such structure, wherein numbers identify an individual turn of wire.
As will be further described later in this disclosure, turns of wire are arranged in columns and rows (or “layers”), and in a coil that uses round wire and has generally perfect nesting, the columns are shifted a half wire diameter, from layer to layer. Because of this column shifting, two layers of wire will take up less space than to two wire diameters, as would be the case with the layers sitting right on top of each other. The image in FIG. 1 illustrates a “perfect nesting” of round wire, and it results in the greatest amount of copper being placed into a given area. This is ideal and highly desirable because such coils will have the lowest electrical resistance and also the lowest thermal resistance, since each turn of wire may be in contact with up to six surrounding turns, resulting in optimal sharing of any heat generated. FIG. 1A illustrates how turns on a typical coil may be arranged. The teachings of the invention observe six surrounding turns form the shape of a hexagon, wherein the hexagonal nature of the turns is illustrated with shading, and will be addressed in greater detail later in this disclosure.
With continued reference to FIG. 1A, it can be seen that turns on the bottom of the figure are arranged side by side in columns. Since there are six turns in the figure, the thickness is simply equal to six wire diameters. As additional layers are added to the coil, they will add 0.866 times a wire diameter in the width to the coil.
The coil illustrated with reference to FIG. 1A may be made using a coil former whose side-walls are set to an integer number of turns on the first layer. When this is done, the number of turns on the next layer will be one less than the first layer (five turns in this illustration), followed by a layer with the same number of turns as for the first layer, and the like as turns are added.
There is another possibility, which is to add a half wire diameter to the integer number for the side-wall distance. In this case, the number of turns on each subsequent layer will be as illustrated with reference to FIG. 1B.
The hexagonal arrangement of turns is clearly visible in both cases, and is highlighted by shading some of the turns, by way of illustrative example.
With continued reference to FIGS. 1A and 1B, it can be seen that top and bottom layers are relatively “flat”, whereas the left and right sides of the coil as herein presented appear “jagged”, due to empty half-turns of wire on the left and right sides of the coil.
There is another possible way to arrange the turns of wire that results in an opposite scenario, wherein the left and right sides form relatively “flat” surfaces, and the top and bottom sides appear somewhat “jagged” due to the empty half-turn areas. This is illustrated with reference to FIG. 1C. By way of example, such an arrangement of turns may be desirable for thin coils and coils where heat must be removed from the left and right sides of the coil. Unfortunately this type of coil is not easily created using conventional winding techniques. During the typical winding process, as a coil is being wound in a conventional spiral manor, placing turns of wire from left-to-right on one layer, followed by right-to-left on the next, makes it difficult indeed to arrange the left-to-right turns to stagger upward and downward. Thus, the conventional coil winding process is limited in this regard.
While taking another look at the two possible ways of arranging turns of wire in which the hexagonal patterns exist, it is to be observed that there is a constant angular relationship of the turns, and of the hexagon. For conventional coils described above, whose turns are arranged to result in a relatively “flat” bottom and top, this relationship puts the angle at 150 degrees with respect to the side wall, as illustrated with reference to FIG. 1D.
By way of further teachings for the alternative coil described above, whose turns are arranged to result in a relatively “flat” left and right side, this relationship puts the angle at 120 degrees with respect to the side wall, as illustrated with reference to FIG. 1E.
With respect to the illustrations above, it is common for coil formers to have a flat “bottom”. That is, all turns on the inner-most radius of the coil are arranged on the same axis and at the same radius. Because of this, if it is desired to create a self-supporting coil, it may be difficult to remove the coil former after the winding process is complete. As turns of wire and layers accumulate, inward forces from each turn press inward on the interior of the former, essentially gripping it. Release agents can be used to aid in removal of the coil from its former, but it would still be more desirable if the coil became separated from the former more easily.
Although the figures discussed above, by way of example, show a cross sectional view of one part of the coil, the degree of nesting cannot be maintained all the way around the entire circumference of the coil. The reason is because the groves formed by the turns on each layer are essentially two-dimensional groves. Sooner or later, at one location around the circumference or another, the turns from each layer must “cross-over” turns from the previous layer.
At the locations where the turns cross-over, there is no longer a space advantage in terms of the reduction in space needed for two layers. At the cross-over locations, the space required truly equals two wire diameters. Likewise, there is no longer the same degree of thermal conductivity at the cross-over locations either, since the contact area, insomuch as the number of places where one turn is in contact with another turn is reduced.
In a most optimal case, all cross-over locations can be restricted to a single location in the wound circumference of the coil. When coils are wound in a typical spiral fashion, where the first turn is located at the inner-most radius of the coil and last turn is located at the outer-most radius, this type of coil winding technique is known as “ortho-cyclic winding.”
Ortho-cyclic wound coils are typically rare indeed because the machines that make them are very specialized, and because such coils must be wound very slowly and precisely. Yet further, for general-purpose applications, the level of copper packing and thermal conductivity are not needed, and thus, the additional cost and time associated with ortho-cyclic coils is avoided.
It is far more often for coils to be “random-wound” or “scramble-wound,” where the cross-over locations appear at randomized locations along the winding circumference. For coils that do not have round interiors, but instead have angles and straight spans, the reduced tension along the straight span coupled with the length of the span will usually allow the cross-over locations to fall along these spans instead of at the curved corners. This is why coils, which start out having angular or non-round interiors, will often wind up having more rounded exteriors.
Since many cross-over locations will fall along the long spans, one effectively ends up with cross-over locations on top of other cross-over locations until the entire exterior is round, at which point the tension is spread evenly along the entire circumference. After that, randomization of the cross-over locations will keep the coil exterior round, as illustrated with reference to FIG. 2.
There is another drawback to conventional coil winding as well as to ortho-cyclic wound coils. Both types start their winding process at the inner-most radius of the coil, and essentially form a spiral outward, as each layer of the coil is added. Thus, one of the coil's lead wires exists on the inside of the coil and the other coil's lead wire exists on the outside. The two separate locations for lead wires may be disadvantageous in circumstances where both lead wires need to be connected on the outside radius of the coil, because in order for the inner-wire to reach the outer circumference, it will need to be lead-out along the side-wall of the coil, effectively adding another wire diameter to the thickness of the coil.
For motors that use a slotted stator, the coils are most often “race-track-shaped,” with the long portion of the coil contained within the slots, and the turn-around areas being folded over the outside of the slots. These turn-around areas are called “end-turns,” as illustrated with reference to FIG. 3.
When creating a coil to be placed into slots, the coil can be created in several different ways. In low-performance motors and actuators, coils are most often “scramble wound”. As mentioned above, with a “scramble wound” coil, the turns that cross-over from column to column will be located at random locations around the winding circumference. Because of this, there will be many areas within the coil which are filled with material other than copper, such as air or insulation, which will exist in the areas where turns are crossing over each other. The randomized cross-over locations will require the coil to be wider, thus diminishing the amount of copper placed into the slots, and also diminishing the heat capacity of the coil due to the random air locations within the coil.
As described above, ortho-cyclic wound coils, coils having restricted cross-over locations, may be used in an effort to maximize the amount of copper within the coil, by restricting the cross-over locations to only a single area of the winding circumference. However, although the cross-over locations may be located in only a single place, the width of the coil will tend to bulge out at the area where cross-over points exist, as illustrated with reference to FIG. 4.
As illustrated with continued reference to FIG. 4, all of the turns are perfectly nested all the way around the circumference of the coil, except at the left/bottom side in this illustration, where all of the cross-over locations reside. This clearly demonstrates that at the cross-over locations the coil must bulge outward (thus the width of the coil must increase) because where wires cross over each other, there is no space advantage. It is clearly observed that the coil begins in the inner-most radius, and forms an outward spiral ending on the outside.
Because of the bulging outward, this necessitates that the outside diameter of the motor be made large enough to accommodate the bulged coil area.
There is another downside to ortho-cyclic coils. Since the cross-over locations effectively contain a lot of air, due to the spacing between turns, the thermal conductivity and heat sharing is dramatically reduced in the cross-over area of the coil. For a high-performance motor, this could impose a performance limit far below the rest of the coil.
By way of example for a motor, actuator or other device that can use a coil having only two columns, another type of coil-winding technique may be used, such as described in U.S. Pat. No. 5,237,165 to Tingley, III for Multi-Turn Coil Structure and Methods of Winding Same, wherein this type of coil places the turns in a side-by-side fashion, and winds both coils on a coil former at the same time. Since both left-half and right-half of the coil are wound at the same time, turn numbers are identified as 1L and 1R for turn number one on the left and right sides respectively; 2L and 2R for turn number two on the left and right sides respectively, etc. In this construction, the windings are crossing over at many points due to the dual-spiral approach, and because of this, the overall width of the coil is equal to two wire diameters. This results in coil packing as illustrated with reference to FIG. 5. While this type of coil will not have any areas that bulge outward, such as that found in an ortho-cyclic coil of FIG. 4, the downside is that with the side-by-side winding, there is an undesirable amount of air inside the coil.
FIG. 6 illustrates a comparison between a two-column coil wound in a side-by-side fashion to an ortho-cyclic two-column coil. As illustrated, the two-column coil is twice as wide as a single wire diameter, but the ortho-cyclic-wound coil is 1.866 times as wide as a single wire diameter. The thickness difference doesn't seem like much of a difference, but with the side-by-side coil, you can see large square-shaped areas that are not filled with copper, as opposed to small, triangular-shaped areas in the case of ortho-cyclic winding. It is clear that a larger space is required for the side-by-side type of coil.
The ortho-cyclic technique absolutely requires that the first layer of turns (or first few layers for coils that are relatively thin) be placed perfectly, thus creating groves for the following layers. Also, the points at which one turn is complete and the next turn begins must be managed very carefully, to help guide the cross-over locations of layers that follow.
Thus, to aid the ortho-cyclic winding process, a new type of coil former is needed that helps to establish desirable locations of the first few layers of coils, and helps to manage the cross-over locations. Moreover, for coil formers that are separated from the coil after winding (by way of example for creating self-supporting coils), it is desirable for the coil former to be easily separated from the coil, as will be illustrated for embodiments herein described according to the teachings of the present invention.
To restate a problem, ortho-cyclic coils are typically difficult to wind, but they do allow maximum copper density almost all around the winding circumference, except at the cross-over location, where the coil dramatically bulges outward. Side-by-side coils do not have any places around the coil where the coil bulges outward, but there is a reduced copper density at all points around the coil, and also minimized wire-to-wire contact, which in turn minimizes thermal conductivity and heat sharing within the coil. And finally, scramble-wound coils cannot be used for very high performance applications.
There is a need for a new type of coil that is easier to wind than typical ortho-cyclic coils, and allows a high copper packing density of ortho-cyclic coils without a dramatic bulging associated with the coils at cross-over locations.